1. Field of the Invention
This invention relates to measurement of particle size distributions via light scattering.
2. Discussion of the Related Art
One class of laser particle measurement instruments, commonly referred to as Ensemble Laser Diffraction (ELD) instruments, use a laser beam to illuminate particles in a measurement volume and an array of sensors to detect the intensities of light scattered from the laser beam by the particles at various scattering angles. The particle size distribution is then calculated using a light scattering function calculated from either Mie theory or Fraunhofer diffraction theory. However, the light scattering function is predicted for a single scattering mode or signature. In applications with moderate-high particulate loading, however, the intensities detected result from multiple scattering events. In order to accurately compute the particle size distribution, the effects of multiple scattering must be taken into account.
Multiple scattering effects have been modeled in (i) "Modeling of Multiple Scattering Effects in Fraunhofer Diffraction Particle Size Analysis" by E. D. Hirleman, Particle Characterization 5, 57-65 (1988), and (ii) "A General Solution to the Inverse Near-forward Scattering Particle Sizing Problem in Multiple Scattering Environments: Theory" by E. D. Hirleman, Proceeding of the 2nd International Congress on Optical Particle Sizing, Mar. 5-8, 1990, pp. 159-168. However, while these articles set forth a general model for characterizing the phenomenon of multiple scattering, they do not provide a method for deriving the single scattering signature without assuming, a priori, the particle size distribution. Such methods are impractical for use in an industrial application, such as one which uses the particle size distribution to achieve a real-time control function. In the prior art, the multiple scattering effect is ignored, leading to significant error in the computed particle size distribution.